Organic Rankine Cycles (ORC) are used to utilize waste heat (e.g., from power generation, technological processes in metal manufacturing, glass production, chemical industry, from compressors, internal combustion engines, etc.). Conventional ORC technology is only able to use a certain amount of waste heat due to the limited thermal stability of organic fluids. Limited thermal stability of organic fluids limits the thermal efficiency of ORC systems if heat source temperature exceeds 250 to 300° C. On average, the total efficiency of ORC units, known from the state of the art, do not exceed values of 10%. Therefore, 90% of thermal energy is wasted to the atmosphere.
The use of Supercritical CO2 (S—CO2) cycles allows waste heat utilization with an efficiency of up to 20% in very compact systems. The size of the system is half of that using standard ORC technology. Compact systems may be used to utilize waste heat from different heat sources.
There are two basic system layouts for S—CO2 cycles known from the state of the art (e.g., regenerative and non-regenerative). The two cycle systems differ from each other by the presence or absence of intermediate heating of cycle fluid by turbine exhaust gases in recuperators.
Both system layouts are used to utilize heat from sources with low power and temperature level with help of ORC and S—CO2 cycles.
The internal thermal efficiency of regenerative cycles is almost twice as high as the efficiency of non-regenerative cycles. Regenerative cycles may exceed 30% for S—CO2 cycle systems. However, in real conditions of S—CO2 cycle implementation net efficiency, the rate of thermal to electrical energy conversion, for systems with simple layouts, is around 10% of total thermal energy supplied by the heat source. To improve the performance and achieve 20% efficiency, more complex system layouts are to be used.
S—CO2 cycle implementation, depending on the environmental conditions and layout, may require both pumps for liquefied CO2 flow and compressors for S—CO2 gas compression. At real conditions, regenerative cycles have more than twice higher internal thermal efficiency than non-regenerative cycles and take less thermal energy from the heat source. Even for relatively low temperatures of heat sources, temperatures at the heater outlet in regenerative cycles remain relatively high. Relatively high temperature allows utilization of remained thermal energy in sequentially located units.
To improve S—CO2 system efficiency, a simple sequential arrangement of at least two independent S—CO2 systems is possible (e.g., in series one after another within a gas flow with waste heat). In the sequential arrangement, the second S—CO2 regenerative cycle utilizes the heat downstream to the first regenerative cycle providing noticeable higher net efficiency of the waste heat utilization arrangement as a whole.
From the state of the art (e.g., WO2012074905A2 and WO2012074911A2), more complex sequential arrangements of two S—CO2 systems are known. The two sequentially arranged regenerative S—CO2 systems in a heat utilizing unit, described in the state of the art, include one common/merged cooler. An advantage is a reduction of components, because only one cooler is required. The system complexity rises and the control gets more complicated because mass flow is to be internally distributed between two turbines and united in a single cooler. In WO2012074905A2, pumps are used assuming liquefied CO2 flow subsequently to the cooler. In WO2012074911A2, compressors are used assuming a supercritical CO2 gas flow subsequently to the cooler.
A further integration is achieved joining the heaters into a single unit (e.g., as described in WO2011119650A2 and O2012074940A3). Both layouts of regenerative S—CO2 systems include two expansion turbines, two recuperators, but just one joint heater, one joint cooler, and one pump for liquid CO2 flow. There are less components than in the systems described above, but the layouts require more complex flow management. Two flow streams are joined at one point of the system and split up back to separate streams at another point of the system upstream.
In WO2011119650A2, the flow stream is split up after a pump, and one flow portion is directly forwarded to a waste heat exchanger. In WO2012074940A3, the flow, before split up, passes through a recuperator placed downstream the pump and only after that the flow portion is entering the waste heat exchanger.
The above described different layouts of S—CO2 system arrangements differ in thermodynamic processes, exhibit different efficiencies, include different hardware components, and demand different system mass flow management and control requirements. A reduction of components requires an increased effort for mass flow management and control. Savings from components lead to increased costs for control and higher complexity with potentially increased error rate.